Pearl River, China
The Pearl River (Zhū Jiāng) is one of the largest rivers in China and the second largest in terms of discharge. The Pearl River delivers each year 89 x 106 tonnes of sediment into the estuary. But as the estuary is closely located to the cities, that has expanded rapidly over the last years, both in terms of population and area, the environmental impact has been significant along the river. As the industries have been trying to keep up with the rapid industrial expansion, trying to lower production costs, the wastewater has been flowing untreated directly into the river and subsiding into the sediments. The annual discharges of waste-and sewage water had reached 200 million and 40 million tons respectively in 1995 (Wong et al.1995). But the industrial waste is not the sole source of pollution, as mining and smelting activities upstream, the agriculture in connection to the Pearl River and atmospheric inputs act as heavy contributors to the pollution. The main pollutants in the Pearl River are Cadmium (Cd), Arsenic (As), Copper (Cu), Lead (Pb) and Zinc (Zn). These are a serious pollutes, as the heavy minerals are very toxic and very persistent to degradation and are very often incorporated into the food webs, which pose as a risk to the local health.
Sediment Distribution and Consequence
Wetlands
The wetlands of the Pearl River delta, in the higher parts, is mainly dominated by a sandy soil, with 60% - 85% of sand and between 13% - 35 % of silt with only >1% clay. This means that the sediments have good permeability, and that the water that percolates in the soil is heavily polluted and that the pollutants spread in the soil. At these sites the main accumulation of Cd and Cu are at a peak at around 40cm depth in the soil, while Pb and Zn are higher concentrations below 40cm depth. This can be an effect of the tidal waves in the Pearl River estuary. As the tidal waves sweep in, it re-suspends the heavy mineral particles and wash the away with the tide water. The accumulation peaks in the sediments can depend on the roots in the wetlands, as they protect the soil from deep erosion during the high tide.
The Estuary
In contrast to the direct and indirectly connected wetlands the estuary itself has a silt-dominated composition. Samples taken and analyzed along the western section of the estuary, in close proximity of the Xiao Island, displayed a composition of 60.7% silt, 38.2% clay and merely 1.1% sand. (Liu et al. 2010). The Area displayed a high sedimentation rate of 3.85cm/a (Lin et al. 1998). The composition and sedimentation rate indicate a local environment with lower energy, allowing the suspended material to subside and quickly form new sediment.
In the top 40-60cm of a core, extracted by Liu, high concentrations of Cu, Ni, Pb, Zn, Cd and Co were present. All Heavy metals (Cr, Cu, Cd) presented concentrations above Class I of the National Standard of PR China (2002) (GB 18668-2002). The concentrations exceeded 9.3, 8.9 and 6,8 times the background concentrations of Hong Kong marine sediments. The core sample indicated stable concentrations 45 cm under the surface and below. In a report by Li et al. (2000a) a core extracted in 1997 indicated no evident concentration of contaminants, with the exception of Pb and Zn. This sudden enrichment of contaminants over the last 15-20 years strongly suggest an anthropogenic influence on the estuary. The low permeability of the clay-silt sediment restricts much of the pollution to the upper regions of the estuary, but simultaneously act as a long-term storage medium causing the sediment to act as a secondary pollution source.
Provenance
The provenance of the sediments that the Pearl River distributes is most easily described as Quaternary sediments. As the Pearl River is a complex of rivers with different tributaries, flowing into and forming the Pearl River, the sediments that is discharged and deposited along the Pearl River derives from a wide variety of the geology of China. The River complex is located in a granite-enriched part of southeastern China. In the western parts of China, the landscape is mostly dominated by mountain ranges, such as the Himalayas.
An analysis done by Zhou Hui et. al. (2011), of the saturation of soft clay, show that the clay in the Pearl River delta is comprised of mostly quartz with approximately 30% - 50% in most parts of the delta, which correlates with the geology in China, being from the Quaternary. The second highest content being Askanite, that is primarily found in Mongolia, indicates a source of a tributary to the Pearl River Complex. The secondary minerals that comprise the clay with ⩽10% is muscovite, kaolinite, feldspar and others with <4% of the total composition. The pores in the clay that is found in the Pearl River delta shows that the soils are dominated by small uniform pores with good connectivity, and as the consolidating pressure increases the pore voids gradually becomes smaller, resulting in lesser permeability. This does not occur at the wetlands higher in the river mouth, as the tide flushes away all the small fragments leaving only the sand. The total amount of k-feldspar in the sediment deposits, and the fact that the pores is uniform in size is an indication that the sediment have been weathered during a long time before deposit and transport.
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